In present-day motor vehicles, air springs are used increasingly as a suspension because they make possible a comfortable suspension as well as a level control of the vehicle. The air springs of the motor vehicle are, preferably, connected via a connecting line to an ancillary volume. The stiffness of the air spring can be adjusted with the aid of this ancillary volume. The connecting line has a large cross section so that air can be exchanged between the air spring and the ancillary volume with an adequately high dynamic. In this way, it is, for example, possible that air can be transferred from the air spring into the ancillary volume so that the air spring can deflect softly even for a short-term shortening of the air spring such as when driving over an obstacle. A valve is connected into the connecting line and this valve blocks the connecting line in its closed position and clears the connecting line in its open position. Preferably, a valve, which can be actuated by air pressure, is used because with a valve of this kind, large cross sections in a connecting line can be switched in a simple manner. During general driving operation of the vehicle, the valve is in the open position so that the air spring is connected continuously to the ancillary volume and so that the air spring can deflect softly thereby ensuring a high degree of suspension comfort of the motor vehicle.
There are, however, driving situations in which a soft comfortable displacement of the air spring is not required; rather, a high stiffness of the air spring is needed. For example, when driving through a curve, the air springs of all wheels should have a high stiffness so that the vehicle does not lean too much when going through the curve. A driving situation of this kind is, for example, detected by sensors and the valve in the connecting line is driven and transferred from the open position into the closed position so that the connecting line is blocked. The air spring is then no longer connected to the ancillary volume so that the air spring exhibits a higher stiffness.
An end of a driving situation which requires a high stiffness of one or several air springs of the motor vehicle, is, in turn, detected by the sensors and the corresponding valves in the connecting lines are again driven so that the valves, in principle, can transfer from the closed position into the open position. Here, and notwithstanding the driving of a valve, the corresponding valve should not transfer into the open position when a high pressure difference between the air spring the ancillary volume is present because, otherwise, this can lead to a spontaneous pressure equalization between the air spring and the ancillary volume because of the large cross section of the connecting line. This can, for example, lead to a spontaneous collapse of the air springs. For example, if the sensors would detect that a motor vehicle goes from movement in a curve to movement along a straight line, then the valves of the wheels are so driven that they, in principle, can transfer from a closed position into an open position. If, at this instant, and because of a leaning of the vehicle still present, there is a still high air pressure in the corresponding air spring, then a spontaneous opening of the valve would lead to a spontaneous pressure equalization between the air spring and the ancillary volume so that the air pressure in the air spring correspondingly drops and the vehicle leans very greatly to one side. For these reasons, the valve should only then open when a small pressure difference is present between the air pressure in the air spring and the air pressure in the ancillary volume. The above description makes clear that the valve in the connecting line is indeed very significant.
German patent publication 1,914,696 discloses an elevation control arrangement for a chassis of a rail vehicle. The vehicle frame of the rail vehicle is suspended by an air spring relative to the undercarriage or bogie. The air spring is connected to an ancillary volume via a connecting line. A valve is mounted in the connecting line and has an open position and a closed position. In the general travel operation of the rail vehicle, the valve is in the closed position and blocks the connecting line between the ancillary volume and the air spring. However, if the rail vehicle travels through a curve, then a leaning of the rail vehicle takes place at the side of the rail vehicle corresponding to the outside of the curve and, as a consequence thereof, the air spring is shortened. Because of this inclination of the rail vehicle, the valve, which is in the connecting line, is driven and transfers perforce from the closed position to the open position so that the ancillary volume is now connected to the air spring and air from the ancillary volume can enter into the air spring. Accordingly, the air pressure in the air spring increases and the rail vehicle is lifted at the side which is at the outside of the curve so that the inclination of the rail vehicle is compensated.
The valve disclosed in German patent publication 1,914,696 includes a complicated configuration and also moves perforce from the closed position into the open position when a large pressure difference is present between the air pressure in the ancillary volume and the air pressure in the air spring. For these reasons, the arrangement disclosed in this patent publication (with an air spring and an ancillary volume and especially the valve known from this arrangement) is not suitable for use in modern motor vehicles.